Magnetron modulation method



Jan. 3, 1939. K. FRITZ 2,142,186

I MAGNETRON MODULATION METHOT' 4 Filed Oct. 12, 1955 g 2 Q Q I s K g i t/JHGE a a iaw i 60/1;

I ljgd 5 i l I E i5 i Q s I l OLTHGE v INVENTOR. KARL FRITZ ATTORNEY.

Patented Jan. 3, 1939 MAGNETRON MODULATION METHOD Karl Fritz, Berlin, Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b. H., Berlin, Germany, a corporation of Germany Application October 12, 1935, Serial No. 44,669 In Germany November 30, 1934 3 Claims.

This invention relates to systems for modulating the output energy of a magnetron oscillator. In the types of magnetron oscillators heretofore known it has been the practice to apply modulations either by way of varying the anode potential with respect to the cathode or by varying the intensity of the magnetic field. Both methods have also been employed simultaneously in certain cases.

According to my invention a novel method of modulating the oscillations is to provide a control grid disposed between the cathode and the segmental anodes of the magnetron tube and to apply modulations in a circuit between the control grid and the cathode, while simultaneously modulating the potentials applied to the output circuit which is connected between the cathode and the segmented anodes.

The use of a control grid in a magnetron discharge tube is unusual because of the fact that the magnetrons heretofore known did not require such a control grid in order to be operated under fairly efficient conditions. A virtual cathode closely adjacent'the segmental anodes results from the influence of a magnetic field. The electrons describe helical paths from the cathode toward segmented anodes. Some of these electrons have just sufficient velocity to graze the inner surfaces of the segmented anodes so that as the electron velocity is varied either by variations in the strength of the magnetic field or by variations in the anode potential, or both, the resultant variations in the amplitude of the output energy can be caused to follow the variations of the modulating energy from any desired source.

A magnetron system employing a control grid between the cathode and the segmented anodes is a device which cannot be compared in its oper-- ation with a so-called Barkhausen-Kurz tube because the electron paths in the two systems are dissimilar. A highly positive control grid with respect to the cathode is required in a Barkhausen-Kurz tube in order to give the electrons sufficientacceleration so that they will for the most part shoot through the mesh of the grid and after retardation in the zone between the control grid and the anode they will, many of them, reverse their direction and either impinge upon the control grid or follow through toward the cathode.

The control grid of my invention has no such function as that of the Barkhausen-Kurz tube, but is employed in conjunction with the employment of other means for modulating the energy of the magnetron tube which otherwise would be oscillatory in its characteristics even in the absence of the control grid.

My invention will now be further described by reference to the accompanying drawing in which Figures 1 and 3 show, respectively, the operating characteristics of a magnetron oscillator, first under previously known conditions and then under conditions such as created in carrying out my invention,

Fig. 2 shows the general circuit scheme of a magnetron oscillator to which my invention pertains, and

Fig. 4 shows a preferred arrangement for modulating the output of the magnetron oscillator.

Fig. 1 shows the modulation characteristic of a well known circuit organization. oscillatory current Is and plate or anode current Ia are plotted as functions of the anode potential, the assumption being that the other working conditions have been adjusted to optimum values. If the point marked as be chosen as the working point on the characteristic, then the tube is incapable of being completely modulated, seeing that in the presence of small radio-frequency currents the thermal load of the anode would become excessive; for anode current Ia. and anode potential Us. rise in the same sense for small oscillatory currents Is- The maximum top line, i. e., unmodulated alternating current power attainable in telegraphic work, is impossible in telephone work. On the contrary, the mean anode current must be diminished to obtain acceptable conditions for anode heating, and as a result the tube can no longer be utilized to the fullest extent.

The left-hand branch of the characteristic connecting oscillatory current and anode voltage cannot be used for modulation whenever the limits of maximum radio-frequency power are to be attained in the presence of optimum operating conditions. For further explanation of this situation, reference is made to Fig. 2, where Al and A2 stand for the anodes, and K for the cathode of a magnetron tube. Suppose that an electron, with, the use of optimum values of anode potential and the magnetic field, though in the absence of a controlling modulator alternating field, covers the orb or path indicated .by S1. When the tube starts to generate oscillations, the electrons will be periodically directed towards anode Al and then towards A2. superimposing upon the anode direct-current voltage Ua, the modulation voltages coming .from a source M, it follows that the active anode potentials will be alternately reinforced and weakened. If the anode potential (in the presence of a constant magnetic field) is appreciably reduced, then the electron paths will be so markedly curved (see curve S2) that the radio-frequency control (alternating) potentials will no longer be able to insure a periodic current transfer from one anode to the other, and the result is an abrupt discontinuance of the oscillations.

According to this invention, combination modulation relative to anode current and anode voltage is to be effected in the magnetron in such a way that the anode potential, as known from the prior art, but at the same time also the anode current (emission current) is modulated, more particularly by the aid of an auxiliary electrode in the direct vicinity of the cathode.

The two modulations must be effected in opposition in order that a modulation characteristic of the kind shown in Fig. 3 may be obtained. The outstanding feature of combination modulation, as here disclosed, is a decrease of the anode current, subject to action of the grid, with rise of the anode potential in such a way that the product Ia.Ua, i. e., the heating of the anode, whether small or large, radio-frequency currents are dealt with, will be approximately constant and as small as feasible.

Combination modulation of this kind further offers the advantage that, with decreasing anode current Ia, the oscillatory current IS likewise decreases somewhat, with the result that the new curve I'/Ua turns out to have a steeper shape than without the additional emission modulation Is/Ua. However, a steeper characteristic requires lower modulation voltage if the same percentage modulation is to be secured, and this is desirable from the viewpoint of satisfactory efficiency.

Fig. 4 illustrates a scheme adapted to carry into practice the modulation method forming the object of this invention. A denotes the segments of a quadri-partite (four split) anode; K is the cathode having the form of a spiral, while H is the auxiliary electrode mounted inside the said spiral electrode and having the form of a linear conductor. Now, according to this invention, not only the anode potential, but at the same time also the anode current, is modulated by a distinct action shifted in phase an angle of 180 degrees, in order that a characteristic of the kind illustrated in Fig. 3 may be secured, these two modulations being accomplished independently of each other in reference to radio-frequency.

The supplementary modulation is obtained by the aid of an electrically controlling auxiliary electrode. Upon the anode potential the modulation voltage derived from a source M is imposed in the normal manner and upon the auxiliary electrode potential in phase opposition. Disregarding the radio-frequency oscillatory action, the modulation circuit represents a scheme which may be likened to a standard three-electrode or triode tube circuit organization comprising a transformer feedback T. In the presence of certain conditions, i. e., when the tube data and working voltages are satisfactory, selfoscillation is feasible in the audio-frequency regenerative circuit. This circumstance may be utilized for various purposes, more particularly for the excitation of an auxiliary wave, or else for the amplification or regeneration of the audiofrequency circuit. If the auxiliary frequency is chosen inside the audible or tonal range, there results a self-modulated transmitter, and this means that the modulation source may be dispensed with. But if the auxiliary frequency is to act as a periodic bias to insure super-regeneration, then the same should be preferably chosen inside the supersonic range.

The idea underlying the present invention is not confined to the examples here shown.

Having thus described my invention, what I claim is:

1. A method of modulating a magnetron discharge tube having a cathode, a control electrode, a plurality of anodes and a magnetic field producing means which comprises the steps of varying the direct current anode potential with respect to the cathode in accordance with said modulations, and varying the potential on said control electrode with respect to said cathode in accordance with said modulations and in opposition to the variations of direct current potential applied to said anodes.

2. Apparatus of the magnetron discharge tube type having a cathode, a control grid, a plurality of anodes and a magnetic field producing means, an input circuit connected between the control grid and cathode, an output circuit connected between the cathode and anodes, means for generating ultra high frequency oscillations in said apparatus, and means for modulating said oscillations, the last said means including connections to the input and output circuits, and means for producing both amplitude and phase differences between the modulating potentials as applied respectively to the input and output circuits.

3. In a system for modulating ultra-high frequency energy, a magnetron discharge tube having a cathode, cylindrically segmented anodes, a control grid, and a magnetic field producing means, an input circuit connected between the control grid and cathode, an output circuit connected between the cathode and anode, a resonant circuit arrangement inter-connecting certain of said anode segments, a source of direct current potential connected between said cathode and the segmented anodes through said resonant circuit arrangement, means including a source of modulations having connections between the cathode and the anode for varying the potential between the anode and cathode in accordance with the modulations, and means including further connections from said source of modulations in series with the cathode and the control grid for causing the amplitude of electron discharge in the magnetron tube to be varied in an opposite sense to that of the potential modulations applied between the cathode and the anode.

KARL FRITZ. 

